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The Gravity Recovery and Interior Laboratory (GRAIL) was an American lunar science mission in NASA's Discovery Program which used high-quality
gravitational field In physics, a gravitational field is a model used to explain the influences that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenome ...
mapping of the
Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
to determine its interior structure. The two small spacecraft GRAIL A (Ebb) and GRAIL B (Flow) were launched on 10 September 2011 aboard a single launch vehicle: the most-powerful configuration of a
Delta II Delta II was an expendable launch system, originally designed and built by McDonnell Douglas. Delta II was part of the Delta rocket family and entered service in 1989. Delta II vehicles included the Delta 6000, and the two later Delta 7000 va ...
, the 7920H-10. GRAIL A separated from the rocket about nine minutes after launch, GRAIL B followed about eight minutes later. They arrived at their orbits around the Moon 25 hours apart. The first probe entered orbit on 31 December 2011 and the second followed on 1 January 2012. The two spacecraft impacted the Lunar surface on December 17, 2012.


Overview

Maria Zuber Maria T. Zuber (born June 27, 1958) is an American geophysicist who is the vice president for research at the Massachusetts Institute of Technology, where she also holds the position of the E. A. Griswold Professor of Geophysics in the Department ...
of the
Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the ...
was GRAIL's principal investigator.
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
's
Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...
managed the project. NASA budgeted US$496 million for the program to include spacecraft and instrument development, launch, mission operations, and science support. Upon launch the spacecraft were named GRAIL A and GRAIL B and a contest was opened to school children to select names. Nearly 900 classrooms from 45 states, Puerto Rico and the District of Columbia, participated in the contest. The winning names, Ebb and Flow, were suggested by 4th grade students at
Emily Dickinson Elementary School Bozeman is a city and the county seat of Gallatin County, Montana, United States. Located in southwest Montana, the 2020 census put Bozeman's population at 53,293, making it the fourth-largest city in Montana. It is the principal city of th ...
in
Bozeman, Montana Bozeman is a city and the county seat of Gallatin County, Montana, United States. Located in southwest Montana, the 2020 census put Bozeman's population at 53,293, making it the fourth-largest city in Montana. It is the principal city of th ...
. Each
spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
transmitted and received
telemetry Telemetry is the in situ data collection, collection of measurements or other data at remote points and their automatic data transmission, transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Gr ...
from the other spacecraft and Earth-based facilities. By measuring the change in distance between the two spacecraft, the gravity field and geological structure of the Moon was obtained. The two spacecraft were able to detect very small changes in the distance between one another. Changes in distance as small as one
micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
were detectable and measurable. The
gravitational field In physics, a gravitational field is a model used to explain the influences that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenome ...
of the Moon was mapped in unprecedented detail.


Objectives

* Map the structure of the lunar crust and
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
* Understand the asymmetric thermal evolution of the Moon * Determine the subsurface structure of
impact basin An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters ...
s and the origin of lunar mascons * Ascertain the temporal evolution of crustal
brecciation Breccia () is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language, in whi ...
and
magmatism Magmatism is the emplacement of magma within and at the surface of the outer layers of a terrestrial planet, which solidifies as igneous rocks. It does so through magmatic activity or igneous activity, the production, intrusion and extrusion of ...
* Constrain the deep interior structure of the Moon * Place limits on the size of the Moon's
inner core Earth's inner core is the innermost geologic layer of planet Earth. It is primarily a solid ball with a radius of about , which is about 20% of Earth's radius or 70% of the Moon's radius. There are no samples of Earth's core accessible for di ...
The data collection phase of the mission lasted from 7 March 2012 to 29 May 2012, for a total of 88 days. A second phase, at a lower altitude, of data collection began 31 August 2012, and was followed by 12 months of data analysis. On 5 December 2012 NASA released a gravity map of the Moon made from GRAIL data. The knowledge acquired will aid understanding of the evolutionary history of the
terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Ve ...
s and computations of lunar orbits.


Spacecraft


Instruments

* Ka band Lunar Gravity Ranging System (LGRS), derived from the
Gravity Recovery and Climate Experiment The Gravity Recovery and Climate Experiment (GRACE) was a joint mission of NASA and the German Aerospace Center (DLR). Twin satellites took detailed measurements of Earth's gravity field anomalies from its launch in March 2002 to the end of its ...
(GRACE) instrument. 90% of the GRACE software was reused for GRAIL. * Radio science beacon (RSB) * Moon Knowledge Acquired by Middle school students (MoonKAM). Each MoonKAM system (one per spacecraft) consists of a digital video controller and four camera heads. Click her

for a MoonKAM photo from lunar orbit.


Propulsion

Thrusters aboard each spacecraft were capable of producing . Each spacecraft was fueled with of
hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine ...
to be used by the thrusters and main engine to enable the spacecraft to enter lunar orbit and transition to the science phase of its mission. The propulsion subsystem consisted of a main fuel tank and a Re-repressurization system which were activated shortly after lunar orbit insertion.


Mission profile


Launch attempts

All times are in EDT ( UTC-4). File:Preparing GRAIL for Launch.jpg, Spacecraft technicians close the payload fairing around GRAIL before launch. File:GRAIL on the Pad.jpg, GRAIL awaits launch at
Cape Canaveral Air Force Station Cape Canaveral Space Force Station (CCSFS) is an installation of the United States Space Force's Space Launch Delta 45, located on Cape Canaveral in Brevard County, Florida. Headquartered at the nearby Patrick Space Force Base, the statio ...
. File:GRAIL Heads to the Moon.jpg, Fire and smoke light up a blue sky as a
United Launch Alliance United Launch Alliance (ULA), legally United Launch Alliance, LLC, is an American spacecraft launch service provider that manufactures and operates a number of rocket vehicles that are capable of launching spacecraft into orbits around Earth, a ...
Delta II Delta II was an expendable launch system, originally designed and built by McDonnell Douglas. Delta II was part of the Delta rocket family and entered service in 1989. Delta II vehicles included the Delta 6000, and the two later Delta 7000 va ...
Heavy rocket propels GRAIL into space.


Transit phase

Unlike the Apollo program missions, which took three days to reach the Moon, GRAIL made use of a three- to four-month low-energy trans-lunar cruise well outside the Moon's orbit and passing near the Sun-Earth Lagrange point L1 before looping back to rendezvous with the Moon. This extended and circuitous trajectory enabled the mission to reduce fuel requirements, protect instruments and reduce the velocity of the two spacecraft at lunar arrival to help achieve the extremely low orbits with separation between the spacecraft (arriving 25 hours apart) of . The very tight tolerances in the flight plan left little room for error correction leading to a launch window lasting one second and providing only two launch opportunities per day.


Science phase

The primary science phase of GRAIL lasted for 88 days, from 7 March 2012 to 29 May 2012. It was followed by a second science phase that ran from 8 Aug 2012 into early Dec 2012. The gravity mapping technique was similar to that used by
Gravity Recovery and Climate Experiment The Gravity Recovery and Climate Experiment (GRACE) was a joint mission of NASA and the German Aerospace Center (DLR). Twin satellites took detailed measurements of Earth's gravity field anomalies from its launch in March 2002 to the end of its ...
(GRACE), and the spacecraft design was based on XSS-11. The orbital insertion dates were (for GRAIL-A) and (for GRAIL-B). The spacecraft were operated over the 88-day acquisition phase, divided into three 27.3 day long
nadir The nadir (, ; ar, نظير, naẓīr, counterpart) is the direction pointing directly ''below'' a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface. The direc ...
-pointed mapping cycles. Twice each day there was an 8-hour pass in view of the
Deep Space Network The NASA Deep Space Network (DSN) is a worldwide Telecommunications network, network of American spacecraft communication ground segment facilities, located in the United States (California), Spain (Madrid), and Australia (Canberra), that suppo ...
for transmission of science and "E/PO MoonKam" data. The first student-requested MoonKam images were taken by Ebb from 2012 March 15-17 and downlinked to Earth March 20. More than 2,700 schools spanning 52 countries were using the MoonKAM cameras. Flow's MoonKam camera captured LRO as it flew by at a distance of about 12 miles (20 km) on May 3. It's the first footage of a moon-orbiting robotic spacecraft taken by another one.


Terminal phase


Final experiment and mission end

At the end of the science phase and a mission extension, the spacecraft were powered down and decommissioned over a five-day period. The spacecraft impacted the lunar surface on December 17, 2012. Both spacecraft impacted an unnamed lunar mountain between
Philolaus Philolaus (; grc, Φιλόλαος, ''Philólaos''; ) was a Greek Pythagorean and pre-Socratic philosopher. He was born in a Greek colony in Italy and migrated to Greece. Philolaus has been called one of three most prominent figures in the Pytha ...
and Mouchez at . Ebb, the lead spacecraft in formation, impacted first. Flow impacted moments later. Each spacecraft was traveling at . A final experiment was conducted during the final days of the mission. Main engines aboard the spacecraft were fired, depleting remaining fuel. Data from that effort will be used by mission planners to validate fuel consumption
computer models Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of, or the outcome of, a real-world or physical system. The reliability of some mathematical models can be deter ...
to improve predictions of fuel needs for future missions. NASA has announced that the crash site will be named after GRAIL collaborator and first American woman in space,
Sally Ride Sally Kristen Ride (May 26, 1951 – July 23, 2012) was an American astronaut and physicist. Born in Los Angeles, she joined NASA in 1978, and in 1983 became the first American woman and the third woman to fly in space, after cosmonauts V ...
.


Results

Gravity passes through matter. In addition to surface mass, a high-resolution gravity field gives a blurred, but useful, look below the surface. Analyses of the GRAIL data have produced a series of scientific results for the Moon. * The resolution of the gravity field has improved by a large amount over pre-GRAIL results. Early analyses gave the
Gravitation of the Moon 300px, Radial gravity anomaly at the surface of the Moon in mGal The acceleration due to gravity on the surface of the Moon is approximately 1.625 m/s2, about 16.6% that on Earth's surface or 0.166 . Over the entire surface, the variation ...
with fields of degree and order 420 and 660. Subsequent analyses have resulted in higher degree and order fields. Maps of the gravity field were made. * The crustal density and porosity were determined. The crust was fragmented by large ancient impacts. * Long narrow linear features were found that are interpreted to be ancient tabular intrusions or dikes formed by magma. *Combining gravity and Lunar Laser Ranging data gives the 3 principal moments of inertia. The moments indicate that a dense core is small. *Combining gravity and lunar
Topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...
, 74 circular impact basins were identified. Strong increases in gravity that are associated with circular impact basins are mascons discovered by Muller and Sjogren. The strongest gravity anomalies are from basins filled with dense mare material, but the strong gravity also requires that the boundary between the crust and denser mantle be warped upward. Where the crust is thicker, there may be no mare fill but the crust-mantle boundary is still warped upward. *The radius, density, and rigidity of interior layers is inferred. *The Orientale basin is the youngest and best-preserved impact basin on the Moon. The gravity field of this 3-ring basin was mapped at high resolution.


See also

*
List of artificial objects on the Moon This is a partial list of artificial materials left on the Moon, many during the missions of the Apollo program. The table below does not include lesser Apollo mission artificial objects, such as a hammer and other tools, retroreflectors, Apollo ...
* Satellite gravimetry *
Selenoid Selenography is the study of the surface and physical features of the Moon (also known as geography of the Moon, or selenodesy). Like geography and areography, selenography is a subdiscipline within the field of planetary science. Historica ...


References


External links


GRAIL: Mission
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

NASA GRAIL (Gravity Recovery and Interior Laboratory)
– mission home page


NASA Science Missions: GRAIL (Gravity Recovery and Interior Laboratory)

NASA 360 New Worlds New Discoveries 2/2
Retrieved 6/3/2011.
Behind The Scenes of My NASA GRAIL Experience – Day One (AM)
{{Use American English, date=January 2014 Discovery Program Space probes launched in 2011 Missions to the Moon Destroyed space probes G Spacecraft that orbited the Moon Spacecraft that impacted the Moon Articles containing video clips 2012 on the Moon Twin satellites Gravimetry